The present invention relates to an ink jet printing head for printing letters, numerals or characters by ejecting drops of ink and more particularly to a drop-on-demand type of ink jet printing head.
An ink jet printing system can provide noiseless and direct printing on paper and can be advantageously used in particular for the printing of Chinese characters which has been increasingly demanded in recent years, and for high quality printing of English characters and numerals. There are various types of ink jet printing systems, e.g., charge control types, field control types, and drop-on-demand types. The drop-on-demand type is the most promising of these because of its simple printing mechanism.
FIG. 1 illustrates a conventional ink jet printing head for a drop-on-demand type printing system, as disclosed in Japanese Examined Patent Publication (Kokoku) No. 54-35937. In FIG. 1, reference numeral 1 designates a substrate, 2 a cover, and 3 a piezoelectric element. The substrate 1 is made from special ceramics. The upper surface of the substrate is provided with a plurality of recess-like nozzles 4 arrayed in a row perpendicular to the direction of travel of the printing head. Also provided are a plurality of recess-like pressure chambers 5 which communicate with the nozzles 4, and a common ink chamber 6 which communicates the the pressure chambers 5 to supply ink. The cover 2 is mounted on the upper surface of the substrate 1 and is provided with an ink filling port 7 for supplying ink into the common ink chamber 6. The piezoelectric elements 3, each being strip-shaped, are mounted on the upper surface of the cover 2 at positions corresponding to the pressure chambers 5. In this construction, the nozzles 4 are arrayed in a row, as described above.
It is difficult to provide the high density array of nozzles required for hig quality printing. More specifically, high quality printing requires that print dots forming a letter be spaced every 0.1 mm, therefore, the nozzles must be spaced every 0.1 mm. However, a nozzle is generally 0.05 to 0.08 mm in width. This means that the sealing portion between the nozzles would have to be very small, i.e., in the range of 0.02 to 0.05 mm. It is not only difficult to manufacture such a structure, but it is also difficult to ensure reliable sealing. Moreover, the pressure chamber 5 must have a large area, as the displacement of the piezoelectric elements 3 caused by the application of voltage must be sufficiently large for the formation of ink drops. Accordingly, as illustrated in FIG. 2, the pressure chambers 5 and the piezoelectric elements 3 are in a radial arrangement, and the pressure chambers 5 are connected to the nozzles 4, arranged with a spacing of 0.1 mm, via the ink passages 8. As can be seen from FIG. 2, the ink passages 8 converge toward the nozzles 4 and, accordingly, are formed so that the widths thereof become narrower approaching the nozzles 4. Due to this construction, the lengths of the nozzles 4, particularly, the length of the nozzle 4 in the central region of the array, are large. This results in an increase in the frictional resistance to the flow of ink in the nozzle and obstructs the formation of the ink drops, thereby making it difficult to realize high quality printing.